The present invention relates to an image sensor composed of a matrix of light sensor circuits, each of which represents a unit pixel and is capable of producing in an photoelectric converting element a sensor current proportional to the quantity of light falling thereon and converting the sensor current into a voltage signal by using a MOS type transistor with a logarithmic output characteristic in a weak inverse state.
Japanese Laid-open Publication of Unexamined Applications KOKAI No. 219443 (1993) and Japanese Publication of Examined Applications KOKOKU No. 46481 (1995) disclose typical conventional MOS transistor type image sensors, in which a light sensor circuit for one pixel comprises, such as shown in FIG. 1 hereof, a photo-diode PD operating as a photoelectric converting element for producing a sensor current proportional to the quantity of incident light Ls falling thereon, a transistor Q1 having a logarithmic output characteristic in a weak inverse state for converting the sensor current produced in the photodiode into a voltage signal Vpd by using the property of its sub-threshold region, a transistor Q2 for amplifying the voltage signal Vpd and a transistor Q3 for outputting a sensor signal in accordance with a timing pulse of a readout signal Vs and which is characterized by its a wide dynamic range obtained by giving the output a logarithmic characteristic, thereby achieving the high sensitivity of detecting a light signal.
However, the prior-art image sensor using light sensor circuits as respective pixels, in which a sensor current proportional to incident light is produced in photo-electric converting element and converted into voltage signal by using a MOS type transistor having logarithmic output characteristic in a weak inverse state, still involves such a problem that it may suffer the occurrence of unwanted afterglow of each pixel with a decreased quantity of incident illumination falling on the photoelectric converting element.
The above-mentioned light sensor circuit can produce a sensor current in the transistor Q1 while a sufficient quantity of light Ls is falling on the photodiode PD and can therefore detect a light signal at a response speed sufficient not to produce an afterimage of the pixel owing to a relatively small value of resistance of the transistor Q1. However, the transistor Q1 is set to operate with resistance increased by one order when a current therein decreases by one order. Therefore, decreasing the current flowing in the transistor Q1 with a decrease in the quantity of incident light Ls falling to the photodiode PD causes the transistor Q1 to rapidly increase its resistance. A time constant of the circuit containing a parasitic capacity C (junction capacity plus wiring stray capacity) of the photodiode PD with the increased resistance is increased to elongate time necessary for removing electric charge accumulated in the parasitic capacity C. As a result, an afterimage can be viewed for a longer duration as the quantity of incident light Ls decreases.
FIG. 5 shows characteristics of a variable voltage signal Vpd when the sensor current in the photodiode PD rapidly changes from a value 1 E−10 A to a value 1 E−15 A in a conventional sensor system.
The diagram indicates that, in case of outputting a sensor signal at an interval of 1/30 seconds, a voltage signal Vpd can not be saturated within the above duration with a sensor current 1 E−12 A corresponding to the decreased quantity of the incident light falling on the photodiode PD. In other words, the time necessary for saturating the voltage signal Vpd is elongated correspondingly with a decreased value of a sensor current with a decreased quantity of the incident light Ls to the photodiode PD.
Therefore, if a sensor signal is output in accordance with timing pulses of a reading signal Vs as shown in FIG. 13, an output appears with such an afterglow that may be of a higher level at an earlier time. In FIG. 13, Vpd′ designates an inverse amplified voltage signal produced by the amplifying transistor Q2 in a conventional sensor system.